A basic purpose of the engine fuel control system is to provide the engine with fuel in a form suitable for combustion and to control the rate of fuel flow for accurate control of engine speed and acceleration. It is known to control engine acceleration by using hydromechanical fuel metering valves which are operated in accordance with fuel control laws and limits based on sensitivity to engine compressor inlet and delivery pressures (P1 and P3) and compressor speed (NH). Pressure related control exhibits good surge recovery characteristics since fuel flow is automatically reduced as P3 drops at the onset of surge. Unfortunately, hydromechanical systems based on this type of fuel control are sensitive to ambient conditions and fuel properties which result in inconsistent engine acceleration performance and maintenance cost penalties due to the need to readjust the fuel system frequently.
It has been proposed to avoid these problems by instituting electronic closed-loop control of actual engine acceleration (dN/dt or NHdot) in accordance with an engine speed schedule designed to avoid surge conditions. Providing the acceleration control is accurate then consistent acceleration performance is achieved under all normal operating conditions. Currently, the earlier hydromechanical fuel control units are retained in case an engine surge should occur and to provide, in the event of failure of the NHdot engine acceleration control loop, emergency acceleration control.
Fuel control systems of this type have an integrating action and in conjunction with their non-linear engine parameter based characteristics offer distinct advantages in the achievement of accurate NHdot control.